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United States Patent |
5,148,259
|
Kato
,   et al.
|
September 15, 1992
|
Semiconductor device having thin film wiring layer of aluminum
containing carbon
Abstract
A semiconductor device comprises one or a plurality of thin film wiring
layers made of aluminum containing carbon, so as to obtain hillock-free
wiring layers. A method of forming the thin film wiring layer employs a
plasma-enhanced chemical vapor deposition or a magnetron-plasma chemical
vapor deposition to form the thin film wiring layer.
Inventors:
|
Kato; Takashi (Sagamihara, JP);
Ito; Takashi (Kawasaki, JP);
Maeda; Mamoru (Tama, JP)
|
Assignee:
|
Fujitsu Limited (Kawasaki, JP)
|
Appl. No.:
|
740872 |
Filed:
|
July 31, 1991 |
Foreign Application Priority Data
| Aug 19, 1986[JP] | 61-193445 |
| Aug 19, 1986[JP] | 61-193446 |
| Aug 19, 1986[JP] | 61-193447 |
Current U.S. Class: |
257/771; 257/E21.295; 257/E23.159; 257/E29.146; 438/642; 438/652; 438/688; 438/937 |
Intern'l Class: |
H01L 023/48 |
Field of Search: |
357/71,67,60
420/548,550
428/651,548
437/197,194,195,957
148/DIG. 140
204/192.15,192.17
|
References Cited
U.S. Patent Documents
1718685 | Jun., 1929 | De Vries et al. | 420/550.
|
4673623 | Jun., 1987 | Gardner et al. | 428/651.
|
4720434 | Jan., 1988 | Kubo et al. | 420/548.
|
4899206 | Feb., 1990 | Sakurai et al. | 357/71.
|
4942451 | Jul., 1990 | Tamaki et al. | 357/67.
|
4990997 | Feb., 1991 | Nishida | 357/71.
|
5018001 | May., 1991 | Kondo et al. | 357/71.
|
5019891 | May., 1991 | Onuki et al. | 357/71.
|
5040048 | Aug., 1991 | Yasue | 357/67.
|
5051812 | Sep., 1991 | Onuki et al. | 357/67.
|
Foreign Patent Documents |
0253299 | Jan., 1988 | EP | 357/71.
|
0657075 | Apr., 1979 | JP | 420/548.
|
58-182821 | Oct., 1983 | JP | 357/71.
|
0165366 | Aug., 1985 | JP | 428/651.
|
0216445 | Sep., 1986 | JP | 437/197.
|
0284948 | Dec., 1986 | JP | 357/70.
|
63-115372 | May., 1988 | JP | 357/71.
|
1-45163 | Feb., 1989 | JP | 357/71.
|
1-64255 | Mar., 1989 | JP | 357/71.
|
0764818 | Jan., 1957 | GB | 420/548.
|
Primary Examiner: Hille; Rolf
Assistant Examiner: Tran; Minhloan
Attorney, Agent or Firm: Staas & Halsey
Parent Case Text
This application is a continuation of application Ser. No. 07/086,623filed
Aug. 18, 1987, now abandoned.
Claims
What is claimed is:
1. A semiconductor device comprising:
a first layer; and
a second layer formed on said first layer,
said second layer being a thin film wiring layer made of aluminum
containing at least carbon, wherein
the aluminum containing carbon has a grain size which is less than or equal
to 100 nm in said second layer.
2. A semiconductor device as claimed in claim 1 in which said first layer
is made of silicon and said second layer further contains silicon.
3. A semiconductor device as claimed in claim 2 in which said second layer
contains 2 or less atomic percent of the silicon.
4. A semiconductor device as claimed in claim 1, further comprising:
a third layer made of a metal and formed on said second layer; and
a fourth layer formed on said third layer, said fourth layer being a thin
film made of aluminum containing carbon.
5. A semiconductor device as claimed in claim 4, wherein the metal of said
third layer is selected from a group including tungsten and titanium.
6. A semiconductor device as claimed in claim 4, wherein said third layer
is made of aluminum containing carbon, said third layer containing an
atomic percent of carbon which is small compared to atomic percents of
carbon contained in said second and fourth layers.
7. A semiconductor device as claimed in claim 4 in which said first layer
is made of silicon and said second layer further contains silicon.
8. A semiconductor device as claimed in claim 4 in which said second layer
contains an atomic percent of carbon greater than an atomic percent of
carbon contained in said third layer.
9. A semiconductor device as claimed in claim 5 in which a plurality of
pairs of said third layer type and said fourth layer type are provided on
said fourth layer in alternate succession so that each third layer type is
sandwiched between two fourth layer types.
10. A semiconductor device as claimed in claim 1, in which grains of said
second layer are generally oriented on a (200) plane.
11. A semiconductor device as claimed in claim 1, in which said second
layer contains 30 or less atomic percent of carbon in a state where carbon
is chemically bonded to aluminum.
Description
BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor devices having
thin film wiring layers and methods of forming thin film wiring layers,
and more particularly to a semiconductor device having a thin film wiring
layer made of aluminum containing carbon and a method of forming a thin
film wiring layer made of aluminum containing carbon.
An integrated circuit is produced by forming elements on a semiconductor
substrate and connecting the elements by a metal thin film wiring layer.
The size of the elements and the wiring layer is reduced so as to increase
the integration density of the integrated circuit, but presently, the
integration density of the integrated circuit is limited by the limit in
reducing the size of the wiring layer.
When the film thickness of the wiring layer is made extremely small, a
disconnection of the wiring layer easily occurs at a stepped portion of
the wiring layer. Furthermore, aluminum is normally used for the wiring
layer, but the electromigration in the aluminum wiring layer increases
with increasing current density and a void is easily generated in a
portion of the aluminum wiring layer where the aluminum atoms lack. A
disconnection occurs at such a portion of the aluminum wiring layer where
the void is generated. On the other hand, a hillock is generated at a
portion of the aluminum wiring layer where there are excessive the
aluminum atoms, and the hillock easily causes a short-circuit between
layers on the semiconductor substrate In addition, when the aluminum
wiring layer is formed on a doped region of a silicon layer, for example,
the aluminum easily diffuses into the doped region in the spike and
short-circuits a junction between the silicon layer and the doped region.
The above described problems of the aluminum wiring layer are all caused by
the fact that migrations easily occur in the case of aluminum atoms. The
migration includes the electromigration and stress migration. While the
electromigration is dependent on the current density, the stress migration
is independent of the current density. A stress acts on the aluminum
wiring layer from one or more layers in contact with the aluminum wiring
layer, and the aluminum atoms are easily moved by this stress at high
temperatures. Hence, when the aluminum wiring layer is cooled after being
heated, a disconnection easily occurs in the aluminum wiring layer due to
the stress. When a semiconductor device is produced, the semiconductor
device is usually subjected to processes at high temperatures, and it is
thus extremely difficult to suppress the stress migration in the aluminum
wiring layer.
Accordingly, attempts have been made to eliminate the above described
problems by using an aluminum alloy for the wiring layer, such as aluminum
containing copper and aluminum containing silicon. However, when the
aluminum containing copper is used as the wiring layer and this wiring
layer is etched by a reactive ion etching (RIE) using chlorine gases, it
has been found that copper residue remains at the surface of the wiring
layer after the RIE and there is a problem in that it is difficult to
remove this copper residue.
On the other hand, problems also occur when the aluminum containing silicon
is used for the wiring layer, although silicon is prevented from diffusing
into the wiring layer when the wiring layer made of the aluminum
containing silicon is formed on a silicon layer. For example, when a
silicon layer has an n.sup.+ -type doped region and the wiring layer also
covers a contact hole located above and exposing the n.sup.+ -type doped
region, a solid phase epitaxial growth of the silicon contained in the
wiring layer occurs on the n.sup.+ -type doped region especially in a
vicinity of the wall of the contact hole. But since this epitaxially grown
silicon is of the p-type, a p-n junction is formed at the contact portion
between the n.sup.+ -type doped region and the wiring layer thereon and
increases the resistance and the contact portion.
In order to reduce the hillock which is generated in the aluminum wiring
layer, there is a conventional semiconductor device having a plurality of
aluminum wiring layers, with a metal layer interposed between two adjacent
aluminum wiring layers. However, it is impossible to completely eliminate
the hillock in the aluminum wiring layer, and the problems described
before occur due to the generation of the hillock.
Therefore, there are demands for a wiring layer which can effectively
contribute to the further increase in the integration density of the
integrated circuit, and a method of forming such a wiring layer.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a
novel and useful semiconductor device having a thin film wiring layer made
of aluminum containing carbon and a method of forming a thin film wiring
layer made of aluminum containing carbon.
Another and more specific object of the present invention is to provide a
semiconductor device having a thin film wiring layer which is made of
aluminum containing carbon. According to the semiconductor device of the
present invention, it is possible to effectively prevent the generation of
hillock and also suppress the generation of electromigration and stress
migration in the wiring layer which has an extremely small film thickness.
Still another object of the present invention is to provide a semiconductor
device having a plurality of thin film wiring layers which are made of
aluminum containing carbon, and a metal layer formed on top and bottom of
each wiring layer. According to the semiconductor device of the present
invention, it is possible to reduce the resistivity of the wiring layer
caused by the carbon contained in the wiring layer.
A further object of the present invention is to provide a method of forming
a thin film wiring layer, in which a plasma-enhanced chemical vapor
deposition (CVD) is used to form the wiring layer which is made of
aluminum containing carbon. According to the method of the present
invention, it is possible to effectively prevent the generation of hillock
and also suppress the generation of electromigration and stress migration
in the wiring layer which has an extremely small film thickness.
Another object of the present invention is to provide a method of forming a
thin film wiring layer, in which a magnetron-plasma CVD is used to form
the wiring layer which is made of aluminum containing carbon. According to
the method of the present invention, it is possible to stably control the
film thickness of the wiring layer.
Other objects and further features of the present invention will be
apparent from the following detailed description when read in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view showing an essential part of a
conventional semiconductor device having a plurality of aluminum wiring
layers;
FIG. 2 is a cross sectional view showing an essential part of a first
embodiment of the semiconductor device having a thin film wiring layer
according to the present invention;
FIG. 3 shows an atomic percent of carbon versus resistivity characteristic
for explaining a first embodiment of the method of forming the thin film
wiring layer according to the present invention;
FIG. 4 generally shows a plasma-enhanced chemical vapor deposition (CVD)
system used in the first embodiment of the method;
FIG. 5 shows an argon sputter etching time versus ion intensity
characteristic;
FIG. 6 shows dilute hydrogen quantity versus deposition rate and
resistivity characteristics;
FIG. 7 shows magnetic field intensity versus resistivity and atomic percent
of carbon characteristics;
FIG. 8 shows a thermal process temperature versus resistivity
characteristic;
FIG. 9 shows RF power versus deposition rate and resistivity
characteristics;
FIGS. 10A and 10B are cross sectional views showing films formed by the
conventional methods;
FIG. 10C is a cross sectional view showing a film formed by the method of
the present invention employing the plasma-enhanced CVD;
FIG. 11 is a cross sectional view showing an essential part of a second
embodiment of the semiconductor device having thin film wiring layers
according to the present invention;
FIG. 12 is a cross sectional view showing a third embodiment of the
semiconductor device having thin film wiring layers according to the
present invention;
FIG. 13 shows an annealing temperature versus reaction layer thickness
characteristic;
FIGS. 14A through 14C are cross sectional views for explaining the method
of forming the thin films of the third embodiment shown in FIG. 12;
FIGS. 15A and 15B are diagrams for explaining electron motion in the
plasma;
FIG. 16 generally shows a magnetron plasma chemical vapor deposition
(MPCVD) system used in a second embodiment of the method of forming the
thin film wiring layer according to the present invention;
FIG. 17 shows a magnetic field intensity distribution on a silicon wafer;
FIGS. 18A and 18B are diagrams showing the kinds of electron motion;
FIG. 19 shows an aluminum film thickness distribution on a silicon wafer;
FIG. 20 shows an RF power density versus deposition rate characteristic;
FIG. 21 shows a trimethyl aluminum (TMA) carrier gas flow rate versus
deposition rate characteristic;
FIG. 22 shows magnetic lines of force generated by a magnetic field applied
over the silicon wafer;
FIG. 23 shows an RF power density versus deposition rate characteristic;
FIG. 24 shows a magnetic field intensity versus resistivity characteristic;
and
FIGS. 25A and 25B show X-ray diffraction angle versus X-ray intensity
characteristics of aluminum films containing carbon formed by the
plasma-enhanced CVD and the MPCVD, respectively, after an annealing
process is carried out.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows an example of the conventional semiconductor device having a
plurality of aluminum wiring layers. In FIG. 1, a plurality of aluminum
wiring layers 1 are formed on a silicon dioxide (SiO.sub.2) oxide layer 3,
with a metal layer 2 formed above and below each aluminum wiring layer 1.
The generation of hillock is reduced to a certain extent compared to the
case where only a single aluminum wiring layer is formed, however, it is
impossible to completely eliminate the hillock in the aluminum wiring
layers 1, and the problems described before occur due to the generation of
the hillock.
The present invention eliminates the problems of the conventional
semiconductor device by forming a wiring layer made of aluminum containing
carbon.
FIG. 2 shows an essential part of a first embodiment of the semiconductor
device having a thin film wiring layer according to the present invention.
In FIG. 2, a semiconductor device comprises a silicon substrate 11, a
diffusion layer 12, an SiO.sub.2 oxide layer 13, and a wiring layer 14.
The wiring layer 14 constitutes an essential part of the present
invention, and this wiring layer is an aluminum film containing carbon.
A description will now be given on the method of mixing carbon to aluminum.
When an aluminum film is doped with carbon by an ion implantation, the
resistivity of the aluminum film gradually increases when 0.1 atomic
percent or more of carbon is implanted into the aluminum film, but the
carbon precipitates out of the aluminum film under a thermal process of
450.degree. C. It may be regarded that the carbon precipitates out of the
aluminum film because there is no chemical bonding between the carbon and
aluminum, and the carbon precipitates out of the aluminum film by the
thermal process over a solubility limit of 0.1 atomic percent of carbon.
Hence, when the carbon is introduced into the aluminum film so that there
is chemical bonding between the carbon and aluminum, the precipitation of
carbon does not occur even after the thermal process. In this case,
however, the aluminum film containing the carbon can no longer be used as
a wiring layer when the atomic percent of carbon exceeds a predetermined
value, because the resistivity increases exponentially when the atomic
percent of carbon exceeds the predetermined value.
FIG. 3 shows the atomic percent of carbon versus resistivity characteristic
before and after the thermal process, where the atomic percent of carbon
(C/(Al+C)) is varied. As may be seen from FIG. 3, there is virtually no
increase in the resistivity for atomic percent of carbon of 20% or less.
It is also seen that the resistivity becomes approximately one-half after
the thermal process at 450.degree. C. for atomic percent of carbon of 20%
or less. For example, the predetermined value of the atomic percent of
carbon is 30% in FIG. 3.
As a result of X-ray measurements, the aluminum film containing carbon has
fine oriented crystal structure, and it may be regarded that the carbon
enters into the grain boundary The grain size of the aluminum film formed
by a plasma-enhanced CVD before being subjected to the thermal process is
in the order of 20 nm. For this reason, it is possible to suppress the
migration of the aluminum atoms even during the thermal process, and the
crystal growth does not occur rapidly. The grain size of the aluminum film
containing carbon is in the order of 50 nm even after being subjected to a
thermal process at 600.degree. C. for 30 minutes. In addition, the
electromigration caused by the increase in the current density is also
suppressed, and there is no generation of hillocks. When the effects of
electromigration are observed in mean time to failure (MTF), it is found
that the serviceable life of the aluminum film containing carbon is longer
by approximately one digit (unit) when compared with that of the
conventional aluminum film. The activation energy level of the MTF is 0.65
eV for the pure aluminum film, while the activation energy level of the
MTF for the aluminum film containing carbon formed by a magnetron plasma
CVD which will be described later is 0.73 eV, and the reliability of the
aluminum film containing carbon is improved compared to the conventional
aluminum film. The hillock is generated after a thermal process at
400.degree. C. in the case of the conventional aluminum film containing no
carbon, but the aluminum film containing carbon has superior
characteristics compared to those of the conventional aluminum film in
that the hillock is not generated even after the thermal process at
600.degree. C.
The chemical bonding between the carbon and aluminum atoms were measured by
an X-ray optoelectronic analyzer, and the complete chemical bonding
between the carbon and aluminum atoms was confirmed. In the present
invention, the aluminum film having the fine crystal structure contains
therein the carbon in a state where the carbon atoms are chemically bonded
to the aluminum atoms. Accordingly, the fact that the grain size of the
aluminum film containing carbon is 100 nm or less is extremely important
in realizing an extre fine wiring layer, especially because the width of
the wiring layer is recently becoming 1 .mu.m or less.
Contacts must be formed when electrically connecting elements formed on a
silicon substrate, but the conventional devices suffer problems in that
the silicon easily diffuses into the aluminum film from the silicon
substrate However, in the present invention, it is possible to prevent the
silicon from diffusing into the aluminum film containing carbon by adding
silicon to the aluminum film containing carbon. Furthermore, the migration
of the silicon atoms is also restricted by the aluminum film containing
carbon and silicon, and there is very little solid phase epitaxial growth
of silicon at the contact portion As a result, the contact resistance is
extremely small. In this case, it is necessary to limit the atomic percent
of silicon under a solubility limit of 2% since the silicon will
precipitate out of the aluminum film containing the carbon when this
solubility limit is exceeded.
Next, a description will be given on the plasma-enhanced CVD (hereinafter
simply referred to as a plasma CVD) which is employed in forming the
aluminum film containing carbon. Generally, when the aluminum film is
formed by the thermal CVD which subjects the source gas to pyrolysis,
substantial surface irregularities are generated on the aluminum film.
Even in the case of the plasma CVD which is employed in the present
invention, the surface irregularities are generated on the aluminum film
when the aluminum film is formed at a temperature over a temperature at
which the pyrolysis occurs. It may be regarded that the surface
irregularities are generated due to the large cohesion of aluminum, and
this phenomenon is inevitable in the thermal CVD.
Accordingly, in the present invention, the plasma CVD is employed to excite
the organic metal gas and the like so as to enhance the chemical bonding,
and the aluminum film is formed at a temperature under the pyrolysis
temperature, that is, at such a temperature that the cohesion of aluminum
does not occur. The plasma CVD is advantageous in that the chemical
bonding between the carbon and aluminum becomes complete by the use of the
plasma.
FIG. 4 generally shows a plasma CVD system used in the first embodiment of
the method of forming the thin film wiring layer according to the present
invention. As shown in FIG. 4, a silicon (Si) wafer 21 is placed within a
parallel plate type plasma chamber 20. An RF generator 22 generates a
signal of 13.56 MHz, and a heater 23 is provided outside the chamber 20 at
a position below the Si wafer 21. An organic metal gas such as trimethyl
aluminum (Al(CH.sub.3).sub.3, TMA) gas is diluted by hydrogen gas and
introduced into the chamber 20 through shower nozzles 24 of the upper
electrode. In this case, the TMA gas is cooled to a temperature below the
fusing point of 15.degree. C., that is, to 5.degree. C., for example.
The argon sputter etching time versus ion intensity characteristic is shown
in FIG. 5. The deposition is carried out under an RF power of 1 kW, a gas
pressure of 4.5 Torr, and a thermal process temperature of 450.degree. C.
for 30 minutes It can be seen from the analyzed results shown in FIG. 5
that before the thermal process, hydrogen is observed in the film as
indicated by a solid line H(b) in addition to carbon indicated by a
phantom line C(b). But after the thermal process at 450.degree. C, the
hydrogen decreases as indicated by a two-dot chain line H(a), while the
carbon is indicated by a one-dot chain line C(a). It may be regarded that
the decrease in the hydrogen is caused by the thermal process which
eliminates CH.sub.3 base which is not decomposed and is included in the
film before the thermal process. For this reason, it is necessary to carry
out a thermal process after the deposition of the film. In FIG. 5, the
ordinate indicates the intensity in arbitrary units, and a range indicated
by a phantom line in the vicinity of the unit 10.sup.3 corresponds to the
measuring limit.
The atomic percent of carbon contained in the aluminum film can be
controlled by varying the RF power and the dilute hydrogen quantity FIG. 6
shows the dilute hydrogen quantity versus deposition rate and resistivity
characteristics. It is seen from FIG. 6 that the resistivity is high and
the deposition rate is small when the dilute hydrogen quantity is not over
a predetermined value. In other words, the dilute hydrogen quantity must
be over approximately 60 times that of the carrier gas (or source gas).
The carrier gas may be other than the organic metal gas (TMA gas), such as
a gas mixture of organic metal gas and silane (SiH.sub.4) gas.
It is possible to apply a magnetic field during the plasma CVD and carry
out a magnetron-plasma CVD so as to magnetically enhance the plasma
reaction and accordingly form a film having an extremely small
resistivity. A detailed description on the magnetron-plasma CVD will be
given later on in the present specification in conjunction with a second
embodiment of the method according to the present invention. The following
Table compares the gas pressure, film thickness, resistivity and atomic
percent of carbon of the films formed by the plasma CVD (PCVD) and the
magnetron-plasma CVD (MPCVD), where the values in brackets indicate values
obtained after a thermal process at 450.degree. C. for 25 minutes.
TABLE
______________________________________
Deposition Method
PCVD MPCVD
Pressure (Torr)
2.3 4.0 2.3 4.0
______________________________________
Film thickness (nm)
100 135 120 110
(75) (120) (120)
(105)
Resistivity (.OMEGA.-cm)
63 23 15 9.0
(15) (9.2) (6.8)
(5.2)
Atomic percent
22 14 5.7 4.1
of carbon (%)
(22) (5.2)
______________________________________
As may be seen from the Table, it is desirable that the gas pressure is
high within such a range that the generation of plasma is stable, and the
resistivity becomes smaller as the magnetic field intensity increases as
shown in FIG. 7. As may be seen from FIG. 7, the atomic percent of carbon
and the resistivity can be made small by controlling the magnetic field
intensity, and the atomic percent of carbon and the resistivity can be
made small especially for the magnetic field intensity of 200 Gauss or
more.
FIG. 8 shows a thermal process temperature versus resistivity
characteristic. As may be seen from FIG. 8, the thermal process at
300.degree. C. or higher and the magnetron-plasma CVD with a magnetic
field intensity of 780 Gauss are effectively in reducing the resistivity.
FIG. 9 shows RF power versus deposition rate and resistivity
characteristics. The deposition rate increases with the increase in the RF
power, but the resistivity undergoes a peculiar variation. In other words,
when the deposition takes place at a low RF power, there is a problem in
that the deposition rate becomes slow and the relative intake of oxygen
increases to increase the resistivity. On the other hand, when the RF
power is excessively high, it is inconvenient in that a polymer is formed
(that is, the atomic percent of carbon is too large). Accordingly, it is
desirable that the RF power is in a range of 300 W to 800 W.
FIGS. 10A and 10B are cross sectional views showing films formed by the
conventional method employing the thermal CVD and the conventional method
employing vapor deposition or sputtering, respectively. When an aluminum
film 30 is formed on a SiO.sub.2 oxide layer 29 which is provided on top
of a Si layer (or substrate) 28 by the thermal CVD, there are substantial
surface irregularities on the aluminum film 30 as shown in FIG. 10A. When
an aluminum film 31 is formed on the SiO.sub.2 oxide layer 29 by a vapor
deposition or sputtering and thereafter subjected to a thermal process at
450.degree. C., a hillock 32 is generated on the aluminum film 31 as shown
in FIG. 10B.
On the other hand, FIG. 10C is a cross sectional view showing a film formed
by the first embodiment of the method of the present invention employing
the plasma CVD. When an aluminum film 33 containing carbon is formed on
the SiO.sub.2 oxide layer 29 by the plasma CVD and preferably by the
magnetron-plasma CVD with a magnetic field intensity of 780 Gauss, no
hillock is generated on the aluminum film 33 containing carbon, and it is
seen that the film 33 is less affected by the migration of the aluminum
atoms.
FIG. 11 shows an essential part of a second embodiment of the semiconductor
device having thin film wiring layers according to the present invention.
In FIG. 11, a plurality of wiring layers 41 made of aluminum containing
carbon are formed on a silicon dioxide (SiO.sub.2) oxide layer 43, with a
metal layer 42 formed on top and bottom of each wiring layer 41. For
example, the atomic percent of carbon contained in the wiring layers 41 is
set in a range of 10% to 30%. The metal layer 42 may be made of aluminum
containing a small atomic percent of carbon in the order of 0.1% or less,
or other metals such as titanium (Ti) and tungsten (W). According to the
present embodiment, it is possible to further reduce the resistivity of
the wiring layers 41 as a whole and also reduce undesirable effects of
stress on the wiring layers 41, in addition to the effects obtainable in
the first embodiment described before.
FIG. 12 shows an essential part of a third embodiment of the semiconductor
device having thin film wiring layers according to the present invention.
In FIG. 12, a semiconductor device comprises a silicon (Si) substrate 46,
a SiO.sub.2 oxide layer 47 formed on the Si substrate 46, a contact hole
47a in the SiO.sub.2 oxide layer 47, first and second wiring layers 48
made of aluminum containing carbon, a metal layer 49 provided between the
first and second wiring layers 48, and a diffusion layer 50 under the
contact hole 47a. The first wiring layer 48 formed directly on the
SiO.sub.2 oxide layer 47 is in contact with the diffusion layer 50 through
the contact hole 47a. The first wiring layer 48 also contains silicon (Si)
so as to prevent the silicon in the Si substrate 46 from diffusing into
the first wiring layer 48 which is made primarily of aluminum. For
example, when the metal layer 49 is made of aluminum containing carbon,
the atomic percent of carbon contained in the first wiring layer 48 is
made large compared to that of the metal layer 49.
FIG. 13 shows an annealing temperature versus reaction layer thickness
characteristic When a film made of pure aluminum is formed on a Si layer,
for example, and is then subjected to an annealing process (30 min), a
reaction layer is formed on the Si layer at annealing temperatures over
approximately 400.degree. C. as shown in FIG. 13. However, when a film
made of aluminum containing carbon is formed on the Si layer and then
subjected to the annealing process (30 min), the reaction layer is only
formed at annealing temperatures of approximately 500.degree. C. or over
as shown in FIG. 13. Accordingly, when the first wiring layer 48 made of
aluminum containing carbon contains an atomic percent of carbon greater
than that contained in the second wiring layer 48, it is possible to
prevent the formation of the reaction layer on the diffusion layer 50 and
also minimize the resistivity of the first and second wiring layers 48 as
a whole, in addition to the effects obtainable in the first embodiment
described before. In the present embodiment, it is of course possible to
provide more than two wiring layers as in the case of the second
embodiment shown in FIG. 11.
Next, a description will be given on the method of forming the thin film
wiring layers in the semiconductor device shown in FIG. 12, by referring
to FIGS. 14A through 14C. In FIGS. 14A through 14C, those parts which are
the same as those corresponding parts in FIG. 12 are designated by the
same reference numerals, and a description thereof will be omitted.
Firstly, in FIG. 14A, the SiO.sub.2 oxide layer 47 is formed on the Si
substrate 46 to a film thickness of 7000 .ANG., by a thermal oxidation or
CVD.
Secondly, in FIG. 14B, the contact hole 47a is formed in the SiO.sub.2
oxide layer 47 by a known patterning process, and impurities are implanted
into the Si substrate 46 through the contact hole 47a and activated so as
to form the diffusion layer 50. In addition, the first wiring layer 48 is
formed on the SiO.sub.2 oxide layer 47 by a plasma CVD to a thickness of
2000.ANG., where the first wiring layer 48 is made of aluminum containing
15 atomic percent of carbon in a state where the carbon is chemically
bonded to the aluminum. The plasma CVD is carried out by a parallel plate
type plasma CVD system at a gas pressure of 2.3 Torr in an RF plasma of
13.56 MHz, so as to deposit aluminum by introducing a gas mixture of TMA
gas and dilute hydrogen into the plasma. It is necessary to keep the
substrate temperature relatively low within a range of 50.degree. C. to
100.degree. C. in order to keep the grain size small.
It is desirable that the first wiring layer 48 making direct contact with
the diffusion layer 50 (that is, the Si substrate 46) additionally
contains 1 to 2 atomic percent of silicon so as to prevent the silicon in
the Si substrate 46 from diffusing into the first wiring layer 48 which is
made primarily of aluminum. The silicon may be introduced into the first
wiring layer 48 by carrying out the plasma CVD with a gas mixture of TMA
gas, hydrogen gas and silane (SiH.sub.4) gas.
Thirdly, the metal layer 49 is formed on the first wiring layer 48 to a
film thickness of 6000.ANG. by a CVD or sputtering, and the second wiring
layer 48 is formed on the metal layer 49 to a thickness of 2000 .ANG.
similarly as in the case of the first wiring layer. It is not essential
that the second wiring layer 48 contains silicon in addition to carbon.
The first embodiment of the method of forming thin film wiring layer
according to the present invention described before especially with
reference to FIG. 4 employs the plasma CVD. However, the plasma CVD
requires a large RF power in order to obtain the high gas pressure that
would result in the decomposition. However, when a large RF power is used,
the silicon wafer is easily damaged Therefore, it is difficult to stably
control the film thickness of the film which is formed by the plasma CVD.
FIGS. 15A and 15B are diagrams for explaining electron motion in the
plasma, where FIG. 15A shows the electron motion for the case where no
magnetic field is applied and FIG. 15B shows the electron motion for the
case where a magnetic field is applied. In the case shown in FIG. 15A, an
electron e in the plasma moves in a direction opposite to that of an
electric field E while colliding with particles A.
On the other hand, in the case shown in FIG. 15B, the electron e undergoes
a circular motion described by an equation re=3.4.times.(.sqroot.V/B)
(cm), where re denotes the Larmor radius (cm), V denotes the electron
energy (eV) and B denotes the magnetic field intensity (Gauss). Hence, a
traveling distance (mean free path) .lambda.e of the electron e becomes
long, and the electron e undergoes more collisions with the particles A.
As a result, there is more excitation of the gas particles, and the plasma
reaction is enhanced.
According to the conventional etching, the gas pressure is in the order of
10.sup.-2 Torr and low. For this reason, the condition .lambda.e >> re is
satisfied, and no peculiar dependencies exist among the gas pressure, the
power and the source (carrier) gas quantity.
But the present inventors have found that in the case of the MPCVD, the gas
pressure is in a range of 0.5 Torr to 5 Torr and is relatively high
compared to the above described value of 10.sup.-2 Torr, and that because
of this relatively high gas pressure, it is possible to stably control the
film thickness of the film which is formed by the MPCVD when a
predetermined condition is satisfied among the gas pressure, the RF power
and the magnetic field intensity.
Accordingly, a description will now be given with respect to the second
embodiment of the method of forming thin film wiring layer according to
the present invention employing the MPCVD. FIG. 16 generally shows an
MPCVD system used in this second embodiment of the method. As shown in
FIG. 16, a silicon (Si) wafer 61 is placed within a parallel plate type
plasma chamber 60. An RF generator 62 generates a signal of 13.56 MHz, and
a heater 63 is provided outside the chamber 60 at a position below the
silicon wafer 61. A magnet 65 is located outside the chamber 60 below the
heater 63. An organic metal gas such as trimethyl aluminum
(Al(CH.sub.3).sub.3, TMA) gas is diluted by hydrogen gas and introduced
into the chamber 60 through shower nozzles 64 of the upper electrode In
this case, the TMA gas is cooled to a temperature below the fusing point
of 15.degree. C., that is, to 5.degree. C., for example.
FIG. 17 shows a magnetic field intensity distribution on the Si wafer 61,
separately for the horizontal component and the vertical component. The
horizontal component is taken along a direction parallel to the top
surface of the Si wafer 61, and the vertical direction is taken along a
direction perpendicular to the top surface of the Si wafer 61. The motion
of the electron e in the plasma due to the magnetic field can roughly be
divided into a helical motion shown in FIG. 18A and a cycloidal motion
shown in FIG. 18B. The helical motion is caused by the vertical magnetic
field component, while the cycloidal motion is caused by the horizontal
magnetic field component when the electron bent by the horizontal magnetic
field component is reflected by the cathode and drifts by itself. In the
case of the magnetic field which is applied so that the magnetic lines of
force do not penetrate the Si wafer 61 as will be described later in
conjunction with FIG. 22, the magnetic field intensity distribution
becomes as shown in FIG. 17, and thus, there is mainly cycloidal motion at
the central portion of the Si wafer 61 and there is mainly helical motion
in portions on the right and left of the central portion of the Si wafer
61.
FIG. 19 shows an aluminum film thickness distribution on the Si wafer 61.
As may be seen from FIG. 19, the deposition rate at the central portion of
the Si wafer 61 decreases as the RF power density decreases, because the
radius of the cycloidal motion of electron increases due to the decrease
in the RF power density and there is essentially no effect of the magnetic
field.
At an RF power density of 1.0 W/cm.sup.2, the effects of the cycloidal
motion and the helical motion of electrons on the deposition become
approximately the same, and it is possible to form a film having a uniform
film thickness. Furthermore, when the RF power density is increased to 1.3
W/cm.sup.2, for example, there is mainly cycloidal motion of the
electrons, and the plasma reaction is enhanced at the central portion of
the Si wafer 61 thereby increasing the deposition rate only at the central
portion of the Si wafer 61. As a result, the RF power density must be set
within a predetermined range in order to form a film having a uniform
thickness.
FIG. 20 shows an RF power density versus deposition rate characteristic.
The decrease in the deposition rate in the high RF power density region is
peculiar to the present embodiment in which the magnetic field is applied
to magnetically enhance the plasma reaction In a most desirable
embodiment, the RF power density is set within a range of 0.5 W/cm.sup.2
to 2.0 W/cm.sup.2, and an optimum range for the deposition of an aluminum
film by use of TMA gas is 1.0 W/cm.sup.2 to 1.5 W/cm.sup.2.
FIG. 21 shows a TMA carrier gas flow rate versus deposition rate
characteristic. This characteristic shown in FIG. 21 is obtained when the
TMA gas is used as the source gas, dilute hydrogen quantity is 1.5 l/min,
RF power density is 1 W/cm.sup.2, the magnetic field intensity is 780
Gauss and the gas pressure is 2.3 Torr. It may be regarded that the
deposition rate decreases at the central portion of the Si wafer 61 for
TMA carrier gas flow rate of 15 ml/min or more, because the mean free path
of the electrons undergoing the cycloidal motion decreases due to the
large molecules of the TMA and the excitation of the TMA gas is
insufficient. In other words, in order to draw out the desired effects of
the applied magnetic field, the RF power density or the magnetic field
intensity must be increased so that the mean free path .lambda.e and the
Raman radius re become approximately the same.
FIG. 22 shows magnetic lines of force generated by a magnetic field applied
over the Si wafer 61, and the so-called planar magnetron is used to apply
the magnetic field. As shown in FIG. 22, the TMA gas is introduced into a
magnetic field indicated by magnetic lines of force 70, along the vertical
direction as indicated by arrows 71, and the magnetic lines of force 70 do
not penetrate the Si wafer 61 but are distributed above the Si wafer 61 in
a loop. In addition, it is important that the magnetic field intensity is
weaker toward the shower nozzles (not shown) through which the TMA gas is
introduced. By taking these measures, the deposition occurs on the surface
of the Si wafer 61 due to the TMA gas which is excited solely by the
cycloidal motion of electrons in the vicinity of the surface of the Si
wafer 61. In other words, the plasma chemical reaction is confined locally
on the surface of the Si wafer 61 by the applied magnetic field. In this
case, since the deposition takes place mainly in the vicinity of the
surface of the Si wafer 61, it is possible to satisfactorily form a
stepped portion of the wiring layer. On the other hand, since the magnetic
field intensity is weaker toward the shower nozzles, it is possible to
delicately excite the TMA gas by the cycloidal motion of electrons in a
first stage and then enhance the reaction in a latter stage.
FIG. 23 shows an RF power density versus deposition rate characteristic.
Because the mean free path of electrons decreases when the gas pressure is
increased, it is impossible to enhance the decomposition of the gas unless
the magnetic field intensity and the RF power density are respectively set
to high values. As may be seen from FIG. 23, in the case where the
magnetic field intensity is constant, the RF power density must be
increased by an amount corresponding to an amount of increase of the gas
pressure.
Therefore, it is seen that the magnetic field intensity or the RF power
density must be set to a value greater than a predetermined value
depending on the gas pressure, since the gas molecules must be excited to
an energy level greater than or equal to an activation energy level at
which the film deposition occurs. In other words, it is necessary to set a
value P determined by (magnetic field intensity).times.(RF power
density)/(gas pressure) to a value which is proportional to the activation
energy level at which the deposition occurs and is within a predetermined
range. Concretely speaking, it is desirable that the magnetic field
intensity is in a range of 200 Gauss to 1500 Gauss, the RF power density
is in a range of 0.5 W/cm.sup.2 to 2.0 W/cm.sup.2 and the gas pressure is
in a range of 1 Torr to 5 Torr, and in this case, the above described
value P determined from the magnetic field intensity, the RF power density
and the gas pressure falls in a range of 20 Gauss.multidot.W/cm.sup.2
.multidot.Torr to 3000 Gauss.multidot.W/cm.sup.2. Torr. FIG. 24 shows a
magnetic field intensity versus resistivity characteristic, and as may be
seen from FIG. 24, the resistivity is small and is in the order of 20
.mu..OMEGA.-cm when the magnetic field intensity is set to a value over
200 Gauss.
Therefore, according to the present embodiment of the method, it is
possible to stably control the film thickness of the wiring layer which is
formed by the MPCVD, and form an extremely thin wiring layer having a
uniform film thickness.
Next, a description will be given on the X-ray diffraction angle versus
X-ray intensity characteristics of aluminum films containing carbon formed
by the plasma CVD and the MPCVD, respectively, after an annealing process
is carried out. FIGS. 25A and 25B show the X-ray diffraction angle versus
X-ray intensity characteristics of aluminum films containing carbon formed
by the plasma CVD and the MPCVD, respectively, after an annealing process
is carried out at 500.degree. C. for 25 minutes. The intensity of the
magnetic field applied in the MPCVD is set to 780 Gauss for the case shown
in FIG. 25B, but the magnetic field intensity may be set to an arbitrary
value over 200 Gauss.
It is seen from FIG. 25A that the grains of the aluminum film containing
carbon formed by the plasma CVD are oriented generally on the (111) plane,
while it is seen from FIG. 25B that the grains of the aluminum film
containing carbon formed by the MPCVD are generally oriented on the (200)
plane. The grain size of the aluminum film containing carbon formed by the
MPCVD is 60 nm or less even after the annealing process is carried out,
and is considerably small compared to that of the conventional aluminum
film.
As described before, the resistivity of the aluminum film containing carbon
formed by the MPCVD is small compared to that formed by the plasma CVD. In
addition, the texture of the aluminum film containing carbon formed by the
MPCVD is fine (dense) compared to that formed by the plasma CVD. As a
result, the reliability of the aluminum film containing carbon formed by
the MPCVD is high compared to that formed by the plasma CVD.
By comparing FIGS. 25A and 25B, it may be regarded that the above described
advantages of the aluminum film containing carbon formed by the MPCVD over
that formed by the plasma CVD are brought about by the grain orientation
on the (200) plane, in addition to the fact that the aluminum film
contains carbon.
Further, the present invention is not limited to these embodiments, but
various variations and modifications may be made without departing from
the scope of the present invention.
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